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Coatings for High-Temperature Structural Materials: Trends and Opportunities Appendix A Testing and Standards Broadly accepted test methods, standards, and specifications are of great value to both vendors and purchasers of coating services and coated products. They are essential communication mechanisms for purchasers to describe the critical aspects of required coatings and for coating suppliers to unambiguously understand the requirements. Coatings are only used in gas turbines after a substantial testing and evaluation program by the engine manufacturer and, where appropriate, by the coating vendor. Testing and analysis procedures are usually developed by the engine manufacturer to: develop procedures, specifications, and controls for the deposition process serve as a quality control measure to ensure that coated products meet specified properties provide data for performance and lifetime prediction by evaluation under conditions simulating engine conditions These procedures are developed by engine manufacturers to address specific conditions expected in each manufacturer's engine. Notable examples of this approach are found in the several types of high-temperature tests developed by engine manufacturers to simulate the corrosive behavior found in specific engines operating under specific conditions. Burner rigs of various designs have evolved at each manufacturer that, through experience, can generate data for corrosion and thermal-shock resistance. Many different methods of testing and analysis have evolved in the research community to develop new coating compositions, microstructures, and processes as well as to understand coating behavior on a fundamental level. The measurement of coating properties must be viewed in the context of a coating/substrate system. Coating compositions and microstructures are complex and become more so during service at high temperature in corrosive environments. Thus the use of material properties for design or lifetime prediction is usually based on the measurement of the coating systems rather than the bulk materials, the properties of which may differ significantly from the same nominal materials present as a coating. While substantial efforts have been made by individual companies and research organizations to develop satisfactory evaluation procedures, relatively few broadly accepted test methods are available for the evaluation of high-temperature coatings. Consequently, much of the data publicly reported consist of measurements that are directly compared with the behavior of widely used materials. This is a conservative approach suitable for the expensive turbines that are expected to have high reliability. However, the need for increased productivity in the materials and gas-turbine fields argues for the use of commonly accepted test methodologies that allow more cost-effective data generation and increased commonality of property specification. Definitions for the several terms used for standards have been developed by the American Society for Testing and Materials (ASTM) and are followed in this discussion. A standard is defined as a rule for an orderly approach to a specific activity, formulated and applied for the benefit and with the cooperation of all concerned. Six types of full consensus standards are identified by the ASTM: Classification. A systematic arrangement or division of materials, products, systems, or services into groups based on similar characteristics (e.g., origin, composition, properties, or use). Guide. A series of options or instructions that does not recommend a specific course of action. Practice. A definitive procedure for performing one or more specific operations or functions that does not produce a test result. Specification. A precise statement of a set of requirements to be satisfied by a material, product, system, or service that also indicates the procedures for determining whether each of the requirements is satisfied. Terminology. A definition or description of terms or an explanation of symbols, abbreviations, or acronyms. Test method. A definitive procedure for the identification, measurement, and evaluation of one or more qualities, characteristics, or properties of a material, product, system, or service that produces a test result. In general, company specifications dominate the commercial market and address characteristics such as composition, microstructure, thickness, and strain-to-failure. The U.S. military has published specifications that address limited aspects of high-temperature coatings, primarily for the thermal spray (plasma and detonation gun) processes. Broadly available standards developed by consensus through
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Coatings for High-Temperature Structural Materials: Trends and Opportunities organizations such as the Society of Automotive Engineers and ASTM address feedstock composition and powder size for thermal spray processes with limited coating property measurements. Foreign standards, notably British and German, primarily address coating thickness with limited attention to physical or mechanical properties. Testing standards are particularly important for thermal barrier coatings (TBCs) in that they include a ceramic layer(s) and the inherent scatter in the mechanical properties of ceramics is accentuated by the complex microstructure produced by thermal spraying or electron-beam physical vapor deposition (EB-PVD). The lack of standard test methods and data analysis and interpretation techniques for relatively fundamental properties (e.g., strength, adhesion and cohesion, strain-to-failure, and ductility) is accompanied, not unexpectedly, by a lack of standards for more complex properties (e.g., thermal shock, fatigue, wear and erosion, corrosion, and toughness). Although basic and applied research has been conducted to understand coating behavior and to relate processing and microstructure and microchemistry to properties and performance, little of this effort has resulted in standards. The necessity to determine accurately appropriate properties for thermal-spray-deposited coatings has been recognized (Berndt et al., 1992; Dapkunas, 1993), but a similar perspective for coatings applied by other processes has not been developed. For TBCs, the lack of understanding of failure mechanisms hinders the identification of required standards. This situation that exists for current superalloy components is also present for future materials such as monolithic ceramics and ceramic-matrix composites, which may require coatings for oxidation protection. The measurement of properties for use in coating micromechanical design is a significant area that has been neglected. Measurements of properties (e.g., modulus of elasticity, coefficient of thermal expansion (CTE), inter-and intragranular strength, and toughness) would be particularly valuable for design of TBCs and the functionally graded materials that are similar in concept. The difficulty of measurement on the micrometer scale required for these materials is largely responsible for this situation. Test techniques currently in development (e.g., nanoindentation) may alleviate this situation. High-temperature coatings have not been the specific subject of standards development, and many of the standards developed for other applications have been used where appropriate. The status of standard testing and analysis procedures varies with the specific aspect of coating technology considered and is summarized in the following section. NOMENCLATURE AND DESIGNATION The description of high-temperature coating types and coating processes lacks well-defined, universally accepted terminology. For example, the same process may be described as pack aluminizing or chemical vapor deposition. Similar ambiguity occurs in the use of the term thermal spraying, which may inclusively refer to all high-temperature, gas-propelled particulate applications to a substrate or specifically to high-velocity, oxygen-fueled deposition. The proprietary, rather than commodity, nature of the coatings industry necessitates that a specific coating be designated by the manufacturer rather than by technical or trade bodies. There are clear benefits for using commonly accepted terms to describe coating processes or attributes, but there does not appear to be value in the development of commonly accepted coating designations. PROCESSING Standards, or more precisely procedures, for processing are generally developed by coating producers and users. However, the reliance of the military on coatings to achieve desired performance objectives has fostered the issue of specifications for both plasma spray and detonation gun deposition processes. ASTM standards developed for the minerals-handling and powder metallurgy industries have been adopted for use where applicable by the plasma spray industry for the analysis of powders. Although company specifications for processes (e.g., EB-PVD or sputter deposition) are used, they have not been adopted as publicly available standards. Furthermore, suitable reference materials are not available for the calibration of analytical instruments necessary for process control (Dapkunas, 1993). MECHANICAL PROPERTIES Mechanical properties, particularly as a function of temperature, are critical to the performance of a coating and consequently are the subject of measurement in the coatings field. The properties of greatest concern for metallic overlay or conversion coatings are ductile-to-brittle transition, fatigue, thermal-shock resistance, adhesion, and strain-to-failure. Test procedures for these properties are generally of the elevated-temperature uniaxial tensile, creep, stress-rupture, or fatigue type developed for metals and alloys. The evaluation of ceramic overlay coatings used for thermal barriers focuses on adhesion to the substrate and cohesion within the coating. Traditional methods for the qualitative evaluation of adhesion (e.g., bending, scratching, or impacting) that were developed for less complex materials (e.g., zinc coatings) are of limited value but are nonetheless included in ASTM, British Standards Institute, and International Standards Organization (ISO) standards. The most commonly accepted adhesion test
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Coatings for High-Temperature Structural Materials: Trends and Opportunities is ASTM C 633-79, the tensile tab test, which is comparable to DIN 50 160-A, AFNOR NF A91-202-79, and JIS H866680 (Berndt, 1990). This technique is limited by the use of an epoxy adhesive grip attachment at test temperatures significantly lower than service temperatures. Brown et al. (1988) have provided a review of methods used to measure the adherence of coatings applied by thermal spray, including flexure and fracture mechanics techniques, and conclude that widely used tests do not provide the information required and that simulation of service conditions is vital. Thermal-shock tests can provide a qualitative measure of adhesion and have been developed for the porcelain-coated steel industry. The importance of determining the mechanical properties of coatings has encouraged the development of compressive, tensile fatigue techniques for coatings removed from substrates (Beardsley, 1992). These methods have not been codified as standards. Recognition of the importance of more subtle properties such as fracture toughness, thermal-shock response, and thermomechanical fatigue has been manifested in research on the modeling of coating behavior and was the subject of a recent conference (Kokini, 1993). However, standards for the measurement of these properties are not available. Hardness is commonly used as a process control measure, and its application to coatings has been recognized in the development of BS 5411-part 6, Vickers and Knoop microhardness, for metallic coatings. Fracture mechanics analysis has been combined with microindentation to measure the fracture toughness of ceramics and has been applied to ceramic coatings (Besich et al., 1993). More recently, instrumented microindenters and nanoindenters that provide data on deformation as a function of load have been developed that can provide a measure of elastic properties. Nanoindentation offers the potential to measure hardness and elastic modulus of specific portions of microstructures as small as several micrometers that can be applied to modeling. None of these latter techniques have been developed into standards. CORROSION Metallic overlay and conversion coatings are used to impart oxidation and hot-corrosion resistance to alloy substrates. Ceramic TBCs are expected to exhibit corrosion resistance and to protect the substrate as well. Corrosion behavior is determined by a variety of static and dynamic tests in environments selected to simulate expected, pertinent aspects of turbine-operating environments. Static tests in furnaces with stagnant or low gas flows provide information on thermodynamic stability and reaction kinetics. Data usually consist of weight changes with time, corrosive penetration determination by metallographic examination, phase-change identification, and corrosive product formation. Dynamic testing is usually conducted in burner rigs operating at atmospheric pressure and gas velocities of less than 100 feet per second. High-velocity and elevated-pressure burner rigs that more closely simulate turbine conditions are used less frequently. Specimen analysis of burner-rig samples usually consists of metallographic examination and compositional analysis of corrosion products. Standards for the conduct of these tests, the specimen types, and the interpretation of data are not available. Nonetheless, significant amounts of research have allowed the comparison of data among the different tests. Recognition of the value of understanding the relationships among the test methodologies for hot corrosion is reflected in the early publication of ASTM STP-421 that provides focused coverage of the tests used in the 1960s (ASTM, 1967). Similar comparisons of more recently developed tests have not been published. The evaluation of corrosion and thermal-shock resistance of TBCs, particularly plasma-sprayed coatings, has been the subject of considerable research. Engine manufacturers and the NASA Lewis Research Center have used burner rigs of various designs for this purpose, and research at the latter institution has highlighted the necessity of well-designed experiments (Miller et al., 1993). EROSION Although test methodologies to evaluate the erosion of coatings and substrate alloys in turbine environments have been used extensively, standards for generally accepted techniques have not been developed. ASTM G76-83 (Standard Practice for Conducting Erosion Tests by Solid Particle Impact Using Gas Jets), for example, uses large particles at low velocities at temperatures of 18-28°C. Typically, evaluation tests for turbine materials are conducted in burner rigs that use particle injection or in high-temperature furnaces using particles entrained in high-velocity gas streams. Specimen evaluation usually consists of a measurement of surface recession and the data provide a ranking of the materials examined. The degree of erosion damage is influenced by particle properties (e.g., size and hardness), impact angle, and velocity. These influences are in addition to coating properties (e.g., hardness and microstructure) that may vary with temperature and long-term exposure to operating conditions. These parametric difficulties may render standard erosion tests that provide data for accurate performance prediction an unrealistic expectation. THERMAL PROPERTIES Thermal properties are a particular concern for ceramic TBCs. Knowledge of thermal conductivity, preferably at the
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Coatings for High-Temperature Structural Materials: Trends and Opportunities temperature of intended use, is important for design and life prediction, while knowledge of thermal expansion coefficients are critical to understanding the adherence to the substrate and stresses in the coating. Thermal conductivity of bulk materials and coatings is routinely measured by the use of a laser flash apparatus that provides a controlled heat input to the front of a sample and measures surface temperature change at the back. Direct measurements are obtained through the use of the guarded hot plate technique that uses a well-insulated apparatus to heat one side of a specimen, while the temperature is measured on the other side. Standards for the guarded hot plate technique (ASTM C-177) and the laser flash (ASTM E-1461) are available for uncoated specimens and may be extended to coated specimens. The ease of the laser flash technique makes it attractive for commercial use and argues for the development of a standard reference material that can be related to results from the guarded hot plate method. Heat transfer through coatings is influenced by the thermal emissivity as well as the thermal conductivity of the coating. Techniques for measuring emissivity are available and will become more important as operating temperatures increase. These techniques are not available as standards for coatings. The thermal comparator method has been used to determine the conductivity of films thinner than 1 micron. This work has shown that these materials can exhibit conductivities as much as two orders of magnitude lower than bulk materials of the same composition and that significant interfacial thermal resistance can develop (Lambropoulos et al., 1993). This observed behavior has implications for the use of thin multilayer TBCs and warrants the evaluation of this class of materials by techniques for which standard methods are available. CTEs can be measured on coatings removed from a substrate using conventional dilatometry, but the graded composition and microstructure of TBCs adds a degree of complexity that can result in specimen bowing. Although bowing may complicate conventional measurements, this phenomena could, in principle, be used as an alternative method to determine coating expansion. Standards for determining the CTE for coatings are unavailable. MICROSTRUCTURE Microstructure of coatings is routinely inspected and analyzed by metallographic preparation of cross sections. Although examination of metallic overlay and conversion coatings is relatively straightforward, graded coatings, which can include porosity and relatively loosely bonded material in the case of plasma-sprayed material, presents more difficulty. Special techniques (e.g., epoxy infiltration) are required to preserve the coating microstructure. Procedures for metallographic preparation and microstructural analysis have been developed by coating producers and users but have not been codified as standard methods. Porosity, manifested as coating density, has significance for coating thermomechanical behavior, corrodent penetration, heat transfer, and centrifugal loading of turbine disks. Standards for measurement of connected porosity by use of the BET gas absorption technique are routinely used (e.g., ASTM C-577-92-refractory permeability). Unconnected porosity is more difficult to measure. Sophisticated techniques (e.g., small-angle neutron scattering) have been used in research to determine pore size and distribution. This technique is not suitable for routine laboratory or production use but may provide a means to synthesize standard materials for calibration of metallographic or other techniques. INTERNATIONAL ACTIVITIES Although the lack of standards for the evaluation of coatings has not impeded the development of new materials for gas turbines, the increased emphasis on international trade may well cast standards in a new light. The greater interest in standards developed by national standards organizations is manifested for materials generally by activities conducted under the auspices of the Versailles Agreement on Materials and Standards that addresses prestandards research. Standards for coatings are addressed specifically by the Committee for European Normalization Technical Committee 184 for Advanced Technical Ceramics (Working Group 5-ceramic coatings). These standards generally correspond to the standards set by national standards organizations. Nations that do not participate in the development of these standards are placed in the unenviable position of having to provide test data according to procedures developed by competitors. Although this is not an insurmountable obstacle to overseas marketing, the early presence of domestic firms in the development of these standards allows the specific concerns of domestic firms to be accorded consideration and shape the standards finally adopted. Participation of U.S. firms in this activity is clearly important for the long-term well-being of the domestic turbine and coatings industries. The development of standards for analysis of coatings can have immediate, positive effects in providing data that are more useful to the turbine industry, but long-term changes that generally affect industry should also be considered. ISO 9000, an international standard for certification that manufacturers have implemented quality assurance procedures, is increasingly required for sales of products. One facet of this procedure is the identification of test and analysis procedures that ensure that products have specified properties. The availability of standards for coating evaluation provides well-accepted criteria for use in the certification procedure. Similarly, for many coating processes, reproduc-
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Coatings for High-Temperature Structural Materials: Trends and Opportunities ible deposition relies on the skill and judgment of equipment operators. Certification of these operators, similar to that required for welders, may be desirable in meeting the requirements of ISO 9000. This requirement has fostered the formation of the National Aerospace and Defense Contractors Accreditation Program, which aims to develop industrywide quality accreditation procedures. The increased exchange of product data among manufacturers has encouraged the development of computer protocols for the exchange of materials information. This activity is conducted under the auspices of ISO Technical Committee 104, subcommittee 4, and is identified informally as STEP (Standard for the Exchange of Product model) and formally as ISO 10303. Completion of the program is expected by the year 2000. Although originally focused on the exchange of data for computer-aided design, a program is in place to allow the exchange of material data from producers to users (Rumble and Carpenter, 1992). In 1994, standards drafted by the committee for the description of materials properties were developed and are expected to be adopted as ISO standards. Application protocols for the exchange of data relative to specific industries are planned, with testing of polymers the likely first use. International commerce in turbine coatings could well be impacted by this data exchange methodology and warrants participation by the domestic turbine and coating manufacturers. SUMMARY Relatively few standard test and analysis techniques have been developed for high-temperature coatings. The coatings community would be well served by the availability of broadly accepted standard measurement techniques for microstructure, mechanical, and thermal properties on scales appropriate for the design and analysis of coating systems. The Committee for European Normalization has identified coatings measurement standards as a topic for development, and it is reasonable to expect that these standards will be incorporated into the ISO code. The interests of producers and users of domestic coatings would be well served by the participation of these parties in the development of standards that reflect their methods of measurement. The current standards for coatings are listed below. CURRENT STANDARDS American Society for Testing and Materials Processing B 212-89 Standard Test Method for Apparent Density of Free-Flowing Metal Powders B 213-90 Standard Test Method for Flow Rate of Metal Powders B 214-92 Standard Test Method for Sieve Analysis of Granular Metal Powders B 215-90 Standard Test Methods of Sampling Finished Lots of Metal Powders C 702-87 Standard Practice for Reducing Field Samples of Aggregate to Testing Size Properties B 571-91 Standard Test Methods for Adhesion of Metallic Coatings (bend, burnishing, chisel-knife, draw, file, grind-saw, heat-quench, impact, peel, push, scribe-grid tests, qualitative only; similar to BS 5411/Part 10) C 177-85 Steady State Heat Flux Measurements and Thermal Transmission Properties by Means of the Guarded-Hot-Plate Apparatus C 313 Adherence of Porcelain to Steel (discontinued) C 633-79 Standard Test Method for Adhesion or Cohesive Strength of Flame Sprayed Coatings (reapproved 1993) C 577-92 Standard Test Method for Permeability of Refractories Other D 4541-85 Standard Test Method for Pull-Off Strength of Coatings Using Portable Adhesion Testers F 692-80 Standard Test Method for Measuring Adhesion Strength of Solderable Films to Substrates (reapproved 1991)
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Coatings for High-Temperature Structural Materials: Trends and Opportunities Society of Automotive Engineers Processing AMS 2435C Detonation Process-Tungsten Carbide/Cobalt Coating AMS 2436B Coating, Aluminum Oxide-Detonation Deposition AMS 2437B Coating, Plasma Spray Deposition AMS 5791 Powder, Plasma Spray, 56. 5Co-25. 5Cr-10. 5Ni-7. 5W AMS 5792 Powder Plasma Spray, 50(88W-12Co) + 35(70Ni-16.5Cr-4Fe-4Si-3.8B) + 15(80Ni-20Al) AMS 5793 Powder, Plasma Spray, (95Ni-5A1) AMS 7875 Chromium Carbide Plus Nickel-Chromium Alloy Powder, 75Cr2C3 + 25 (80Ni-20Cr Alloy) AMS 7878 Tungsten Carbide Powder, Cobalt Coated AMS 7879 Tungsten Carbide-Cobalt Powder, Cast and Crushed AMS 7880 Tungsten Carbide-Cobalt Powder, Sintered and Crushed Military Standards Processing MIL-C-52023 Coating: Ceramic, Refractory, for High Temperature Protection of Low Carbon Steel -1958 MIL-STD-1886 (AT) Tungsten Carbide-Cobalt Coating, Detonation Process -1992 MIL-C-81751B Coating, Metallic Ceramic MIL-M-80141C Metallizing Outfits, Powder Guns and Accessories -1987 MIL-STD-1884A (AT) Coating, Plasma Spray Deposition -1991 MIL-Z-81572 (AS) Zirconium Oxide, Lime Stabilized, Powder and Rod, for Flame Spraying -1991 MIL-83348 Powders, Plasma Spray (CANCELED) British Standards Institute Properties BS 5411 Methods of Test for Metallic and Related Coatings Part 1 Definitions and Conventions Concerning the Measurements of Thickness Part 2 Review of Methods for the Measurement of Thickness Part 3 Eddy Current Method for Measurement of Thickness of Non-Conductive Coatings on Non-Magnetic Basis Materials Part 4 Coulometric Method for the Measurement of Coating Thickness Part 5 Measurement of Local Thickness Part 6 Vickers and Knoop Microhardness Tests Part 7 Profilometric Method for Measurements of Coating Thickness Part 8 Measurement of Coating Thickness of Metallic Coatings: X-Ray Spectrometric Methods Part 9 Measurement of Coating Thickness of Electrodeposited Nickel Coatings on Magnetic and Non-Magnetic Substrates-Magnetic Method Part 10 1981/ISO 2819-1980 Review of methods for testing adhesion of electrodeposited and chemically deposited metallic coatings on metallic substrates (burnishing, ball burnishing, shot peening, peel [less than 125 microns], file, grinding and sawing, chisel [greater than 125 microns], scribe and grid, bending, twisting, tensile, thermal shock, drawing, cathodic; qualitative only) Part 11 Measurement of Coating Thickness of Non-Magnetic Metallic and Vitreous or Porcelain Enamel Coatings on Magnetic Basis Metals: Magnetic Method Part 12 Beta Backscatter Method for Measurement of Thickness Part 13 Chromate Conversion Coatings on Zinc and Cadmium
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Coatings for High-Temperature Structural Materials: Trends and Opportunities Part 14 Gravimetric Method for Determination of Coating Mass per Unit Area of Conversion Coatings on Metallic Materials Part 15 Review of Methods of Measurement of Ductility Part 16 Scanning Electron Microscopy Method for Measurement of Local Thickness of Coatings by Examination of Cross Sections BSI M. 40 Aerospace Series-Methods for Measuring Coating Thickness by Non-Destructive Testing DIN Processing 50961-87 Electroplated Coatings: Zinc and Chromate Coatings on Iron and Steel: Chromate Treatment of Zinc and Cadmium Coatings 50966-88 Electroplated Coatings: Autocatalytic Nickel-Phosphorus Coatings on Metal in Technical Applications 50967-91 Electrodeposited Coatings of Nickel Plus Chromium and Copper Plus Nickel Plus Chromium 50968-91 Electrodeposited Coatings of Nickel and Nickel Plus Copper Properties 50933-87 Measurement of Coating Thickness by Differential Measurement Using a Stylus 50949-84 Non-Destructive Testing of Anodic Oxidation Coatings on Pure Aluminum and Aluminum Alloys by Measurement of Admittance 50955-83 Measurement of Coating Thickness. Measurement of Thickness of Metallic Coatings by Local Anodic Dissolution: Coulometric Method 50976-89 Corrosion Protection: Hot Dip Batch Galvanizing: Requirements and Testing 50978-85 Testing of Metallic Coatings: Adherence of Hot Dip Zinc Coatings [up to 150 microns] 50982 PT 1-87 Principles of Coating Thickness Measurement: Terminology Associated with Coating Thickness and Measuring Areas 50982 PT 2-87 Principles of Coating Thickness Measurement: Review of Commonly Used Methods of Measurement 50982 PT 3-87 Principles of Coating Thickness Measurement: Selection Criteria and Basic Measurement Procedures 50987-87 Measurement of Coating Thickness by the X-Ray Spectrometric Method 50160-A Tensile Adhesion Other 50960 PT 1-86 Electroplated and Chemical Coatings: Designation and Specification in Technical Documents 50960 PT 2-86 Electroplated and Chemical Coatings: Indications on Drawings Japan Institute of Standards Properties H8666-90 Testing Methods for Thermal Sprayed Ceramic Coatings R4204 Method of Testing Ceramic Coating
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Coatings for High-Temperature Structural Materials: Trends and Opportunities REFERENCES ASTM (American Society for Testing and Materials). 1967. Hot Corrosion Problems Associated with Gas Turbines. Special Technical Publication 421. Philadelphia, Pennsylvania: ASTM. Beardsley, M.B. 1992. Thick thermal barrier coatings. Pp. 567-572 in Proceedings of the Annual Automotive Technology Development Contractors' Coordination Meeting, Dearborn, Michigan, November 2-5. Warrendale, Pennsylvania: Society of Automotive Engineers. Berndt, C.C. 1990. Tensile adhesion testing methodology for thermally sprayed coatings . Journal of Materials Engineering 12:151-158. Berndt, C.C., W. Brindley, A.N. Goland, H. Herman, D.L. Houck, K. Jones, R.A. Miller, R. Neiser, S. Sampath, M. Smith, and P. Spanne. 1992. Current problems in plasma spray processing. Journal of Thermal Spray Technology 1(4):341-356. Besich, G.K., C.W. Florey, F.J. Worzala, and W.J. Lenling. 1993. Fracture toughness of thermal spray ceramic coatings determined by the indentation technique. Journal of Thermal Spray Technology 2(1):35-38. Brown, S.D., B.A. Chapman, and G.B. Wirth. 1988. Fracture kinetics and the mechanical measurement of adherence. Pp. 147-157 in Proceedings of the National Thermal Spray Conference, October 24-27, D. Hauck, ed. Metals Park, Ohio: ASM International. Dapkunas, S.J. 1993. NIST-Industry Workshop on Thermal Spray Coatings Research. NIST Journal of Research 98(3):383-389. Kokini, K., ed. 1993. Ceramic coatings. In Proceedings of the 1993 ASME Winter Annual Meeting, Materials Division, New Orleans, Louisiana, November 28-December 3. New York: American Society of Mechanical Engineers. Lambropoulos, J.C., S.D. Jacobs, S.J. Burns, L. ShawKlein, and S.S. Hwang. 1993. Thermal conductivity of thin films: measurement and microstructural effects. Pp. 21-32 in Thin Film Heat Transfer-Properties and Processing, Vol. 184, M.K. Alam, M.I. Slik, G.P. Grigoropoulos, J.A.C. Humphrey, R.L. Mahajan, and V. Prasad, eds. New York: American Society of Mechanical Engineers. Miller, R.A., G.W. Lissler, and J.M. Jobe. 1993. Characterization and Durability of Plasma Sprayed Zirconia-Yttria and Hafnia-Yttria Thermal Barrier Coatings. NASA Technical Paper 3295. Washington, D.C.: National Aeronautics and Space Administration. Rumble, J., and J. Carpenter. 1992. Materials STEP into the future. Advanced Materials and Processes 142(4):23-27.
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